1. Field of the Invention
The field of art to which this invention pertains is crystalline aluminosilicate production. More specifically this invention relates to a process for modifying the characteristics of a precursor mass comprising sodium X or sodium Y zeolite and amorphous material as a binder to produce an absorbent having characteristics desirable for use in a process for separating olefins from a hydrocarbon feed mixture containing olefins.
2. Description of the Prior Art
That the properties of zeolites can be modified by treating them with certain substances has generally been recognized by the prior art. For instances, U.S. Pat. No. 3,106,593 teaches the use of nitrogen-containing bases such as ammonia and various amines to neutralize surface acidity of certain zeolites used to separate olefins; U.S. Pat. No. 3,698,157 teaches that contacting ZSM-5 and ZSM-8 zeolites with organic-radical substituted silanes improves the selectivity of the zeolites for para-xylene with respect to the other xylenes when used in a xylene separation process; U.S. Pat. No. 3,855,333 teaches that contacting X or Y zeolites with certain alcohols improves the selectivity of the zeolite for the para-isomer with respect to other isomers when used in a para-isomer separation process. More specifically, the prior art has also recognized that treating certain zeolites with caustic solutions will modify particular zeolite properties and produce certain results depending on the type of zeolite and the treating conditions employed.
U.S. Pat. No. 3,326,797, for example, discloses treating high-silica zeolites, having silica to alumina mole ratios between about 6 and 12 (for example, mordenite) with caustic solutions for the purpose of removing a certain percentage of structural silica from the zeolites. The caustic treatment of such a high-silica zeolite, at conditions to preferably retain a final SiO.sub.2 /Al.sub.2 O.sub.3 ratio greater than about 5.5, is found to increase the adsorptive capacity of the zeolite and to increase its catalytic activity particularly when used as a catalyst in such acid-catalyzed reactions such as cracking. The caustic treating of this patent is concerned with etching or leaching of silica from the particular zeolite structure to achieve these characteristics and does not disclose or suggest the addition of alkali metal cations to the zeolite structure during the treating process.
U.S. Pat. No. 3,717,572 discloses treating type A zeolites with nitrogen-free base solutions to modify "surface acidity" of the zeolite when used in a process for separating straight-chain olefins from branch-chain olefins. In contrast to the high-silica zeolites of U.S. Pat. No. 3,326,797, the type A zeolite is characterized by a rather low silica to alumina mole ratio, as taught in U.S. Pat. No. 2,882,243, within the range of 1.85.+-.0.5. The treatment of such zeolites appears to be merely an acid-base reaction to neutralize surface acidity; the base appears not critical and can be generally bases whose cation is an alkaline metal, for instance, soda, potash, and lithium hydroxide. Treatment conditions are such that the concentration of the base solution after impregnation is about the same as the fresh base solution. The result desired and that produced with the particular zeolite is only neutralization of acidity; indeed, the preferred solvent for the base solution is an organic medium (in particular methanol) since in such cases neutralization is obtained while maintaining a practically unchanged adsorptive capacity of the sieve. Neutralization by aqueous base solutions actually lowers this capacity.
Thus the prior art has recognized that similar base treatments of different zeolites can produce different results. Such is not surprising since the zeolites vary considerably in such chemical and physical properties as composition, silica to alumina mole ratio, symmetry, cell dimensions, structure, density, pore diameter, and surface area.
The method of our invention is concerned with the treatment of a precursor mass comprising sodium X or sodium Y zeolite and amorphous material as a binder with an aqueous caustic solution. We have found that the treatment of such precursor mass at particular treatment conditions will produce an absorbent having increased capacity for olefins and decreased catalytic activity when used in a process for separating olefins from paraffins. The absorbent produced is more efficient for olefin separation because of its increased capacity and has longer effective life because of its reduced catalytic activity.